fbpx

 

I'm on a bit of a mission to understand the Rotax electrical system in depth so I've looked for circuit diagrams and read the manuals inside and out.

The RR is very simple. It rectifies the AC from the stators and connects the rectified current to the bus directly if the voltage on C is between 3 and 14 volts. With a capacitor or battery to smooth the pulses, it will be outputting roughly 14 V at all times as long as the generator is spinning fast enough.

When the Ducati RR is powered but there is no capacitive load (no battery or capacitor) then the regulator can self-destruct. Why does this happen? Wouldn't the output voltage spike as the no-load stator windings build voltage, until it exceeds 14 V on the C pin (taking milliseconds), then the SCRs should disconnect from the generator, and the C pin should go to 0. Now the RR should be off because 0 < 3.

But if it destroys itself then that must be wrong. I assume it's some sort of oscillation that occurs because there is no more "smoothing" of the voltage, and the voltage alternates rapidly between only two extremes as it tries and fails to make it settle, and somehow that causes damage.

So what really causes damage when the RR is disconnected from a battery, and has no capacitor and no capacitive load?

  • Re: What exactly destroys the RR when there is no battery?

    by » one year ago


    I think you understand the operation of the regulator quite well.
    I don't think the regulator is going to have any problem with the unstable voltage.
    Depending on the capacitance load of the avionics, It will either oscillate from near zero to the full output which can be 50+v or it will just shut down as you suspect.
    The problem will be if the avionics can't tolerate 50V Alternating DC at tens to hundreds of kilohertz.
    I would be surprised if the Regulator died, but losing one or more of the avionics packages would not be out of the question.
    I do not suggest trying this at home but I suspect there is a bit of Old Wives Tale involved here.
    You are not supposed to ask WHY; You are just supposed to do as instructed Grasshopper.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: What exactly destroys the RR when there is no battery?

    by » one year ago


    The frequency wouldn't be in kilohertz, probably no more than 800 Hz! Because the SCRs once triggered will stay on until the sinusoidal voltage drops back to zero, so they can't be pulsed any faster than the AC frequency. It doesn't have a full-bridge rectifier and then smoothing, and then regulation by pulsing. The rectification and regulation are done together (It's dual half-wave rectification done by two MR2510 diodes and the two TP154E SCRs, and the regulation is performed only by the SCRs).

    So at maximum output, the SCRs would be triggered immediately (minimum output would be not triggering at all even at the peak of the sine). As I look at the schematic, it looks like it's most similar to a triac-based light bulb dimmer. It isn't turning on and off, but it's triggering the SCR earlier in the sine wave, letting more pass through. One of the image attachments shows this, except with a potentiometer instead of the complex circuits for sensing voltage on C. Is that correct? Or does it not adjust its trigger point at all and only lets through a wave or blocks it entirely?

     

    A low capacitance load would damage avionics like you say, but I kept hearing that the Ducati could be damaged and that's why the big capacitor is needed for LFP batteries (to protect the regulator from self-destruction if the BMS isolates the battery). One of the selling points of the Silent Hektik is that it is not damaged under no load (I think this is because it is a hybrid series and shunt regulator and if the series regulator isn't able to reduce the voltage by pulsing, then the shunt kicks in).

    In the first attachment for the Ducati schematic (found on https://www.circuitsonline.net/forum/view/135679), if you get that pulsing 50 V on the C pin (labeled "+12 volts from headlight circuit") would subject the 220 uF capacitor and the Q1 transistor's base to high voltage, but Q1 is a generic high-voltage PNP. I don't think any of those components would be damaged.

    When I get my hands on a spare Ducati that I don't mind never putting back into a plane, I'll hook it up to a variac and oscilloscope and experiment with its failure modes under various loads.

    35231_2_ducati regulator schematic.jpeg (You do not have access to download this file.)
    35231_2_scr trip point.jpg (You do not have access to download this file.)

  • Re: What exactly destroys the RR when there is no battery?

    by » one year ago


    A forum member on an Italian forum claimed that from experience, "senza carico il regolatore muore in pochi secondi" (translated: without a load, the regulator dies in seconds). I think I might try this while connected to monitoring equipment and then open it and see which component failed.


  • Re: What exactly destroys the RR when there is no battery?

    by » one year ago


    As the voltage on the C pin rises the wattage on the resistors and zeners in the circuit also increases. Above 20 volts the applied wattage exceeds the component rating. 


    Thank you said by: Joe

You do not have permissions to reply to this topic.